Escherichia coli is an extremely versatile organism. In addition to being a member of the normal intestinal flora, strains of E. coli also cause bladder infections, meningitis, and diarrhea. Diarrheagenic E. coli include at least five types of E. coli, which cause various symptoms ranging from cholera-like diarrhea to extreme colitis. Each type of diarrheagenic E. coli possesses a particular set of virulence factors, including adhesins, invasins, and/or toxins, which are responsible for causing a specific type of diarrhea.
Enteropathogenic E. coli (EPEC), is a predominant cause of infantile diarrhea worldwide. EPEC disease is characterized by watery diarrhea of varying severity, with vomiting and fever often accompanying the fluid loss. In addition to isolated outbreaks in daycares and nurseries in developed countries, EPEC poses a major endemic health threat to young children (<6 months) in developing countries. Worldwide, EPEC is the leading cause of bacterial mediated diarrhea in young children, and it has been estimated that EPEC kills several hundred thousand children per year.
Enterohemorrhagic E. coli (EHEC), also called Shiga toxin producing E. coli (STEC) or Vero toxin producing E. coli (VTEC), causes a more severe diarrhea than EPEC (enteric colitis) and in approximately 10% of cases, this disease progresses to an often fatal kidney disease, hemolytic uremic syndrome (HUS). EHEC O157:H7 is the most common serotype in Canada and the United States, and is associated with food and water poisoning (3). Other serotypes of EHEC also cause significant problems in Asia, Europe, and South America, and to a lesser extent in North America. EHEC colonizes cattle and causes A/E lesions, but does not cause disease in adult animals, and instead sheds organisms into the environment. This however causes serious health problems as a relatively few EHEC are necessary to infect humans.
Unlike other E. coli diarrheas, such as enterotoxigenic E. coli, diarrhea caused by EHEC and EPEC is not mediated by a toxin. Instead, EPEC and EHEC bind to intestinal surfaces (EPEC the small bowel, EHEC the large bowel) and cause a characteristic histological lesion, called the attaching and effacing (A/E) lesion (8). A/E lesions are marked by dissolution of the intestinal brush border surface and loss of epithelial microvilli (effacement) at the sites of bacterial attachment. Once bound, bacteria reside upon cup-like projections or pedestals. Underlying this pedestal in the epithelial cell are several cytoskeletal components, including actin and actin associated cytoskeletal proteins. Formation of A/E lesions and actin-rich pedestals beneath attaching bacteria is the histopathological hallmark of A/E pathogens (1, 2). This pathology can be recapitulated in cultured cells in vitro, and pedestal formation can be viewed by a fluorescent actin staining assay (2, 11). Formation of the A/E lesion may be responsible for disruption of the brush border and microvilli, fluid secretion, and diarrhea.
EPEC and EHEC belong to a family of A/E pathogens, including several EPEC-like animal pathogens that cause disease in rabbits (REPEC), pigs (PEPEC), and mice (Citrobacter rodentium). These pathogens contain pathogenicity islands (PAIs) that encode specialized secretion systems and secreted virulence factors critical for disease. The genes required for the formation of A/E lesions are thought to be clustered together in a single chromosomal pathogenicity island known as the locus for enterocyte effacement (LEE), which includes regulatory elements, a type III secretion system (TTSS), secreted effector proteins, and their cognate chaperones (4-8).
The LEE contains 41 genes, making it one of the more complex PAIs. The main function of the LEE TTSS is to deliver effectors into host cells, where they subvert host cell functions and mediate disease (9, 10, 34). Five LEE-encoded effectors (Tir, EspG, EspF, Map, and EspH) have been identified (35-40). Tir (for translocated intimin receptor) is translocated into host cells where it binds host cytoskeletal and signaling proteins and initiates actin polymerization at the site of bacterial attachment (31, 44), resulting in formation of actin pedestal structures underneath adherent bacteria, which directly interact with the extracellular loop of Tir via the bacterial outer membrane protein intimin. CesT plays a role as a chaperone for Tir stability and secretion (18, 19).
Four other LEE-encoded TTSS-translocated effectors have been characterized in A/E pathogens: EspH enhances elongation of actin pedestals (40); EspF plays a role in disassembly of tight junctions between intestinal epithelial cells (38); EspG is related to the Shigella microtubule-binding effector VirA (36, 55); and Map localizes to mitochondria (37), but also has a role in actin dynamics (48). Ler (for LEE encoded regulator) is the only LEE encoded regulator identified.